This invention relates to a transfer case for use in the drive train of a vehicle such as an automobile or truck.
Basically, such vehicles have three different types of drive trains. The most common is a two-wheel drive train in which either the two rear wheels or the two front wheels are driven in order to propel the vehicle. While this systems offers the best fuel economy, it has poor traction under rainy, snowy or muddy road conditions.
Another system is a four-wheel drive train in which one drive shaft from a transfer case acts through a rear axle differential to drive the rear wheels while a second drive shaft from the transfer case acts through a front axle differential to drive the front wheels. By disengaging a clutch, one set of wheels may be selectively removed from the drive so as to increase fuel economy.
A four-wheel drive train is particularly advantageous if the vehicle becomes stuck in mud or snow or if the vehicle is driven on slick roads since the front and rear axle differentials act independently of one another and thus one set of wheels may maintain full traction even if one wheel of the other set is slipping. A four-wheel drive system is disadvantageous, however, under normal dry road conditions since, in the four-wheel drive mode, the drive shafts to the two sets of wheels are in effect a single rigid shaft and cannot accommodate differences in speeds between the front wheels and the rear wheels. As a practical matter, the four-wheel mode should only be used under wet or slippery conditions where the road surface has a low coefficient of friction to enable slippage of the wheels to accommodate front and rear wheel speed differences.
A more recent system is referred to as an all-wheel drive system. In such a system, a center differential is located in the transfer case between the front and rear drive shafts and accommodates differences in speeds between the front wheels and the rear wheels. As a result, the system may be kept in an all-wheel drive mode all of the time to provide enhanced control over the vehicle. If the vehicle is operating on extremely slippery roads or becomes stuck, the center differential may be locked out by means of a clutch in order to convert the system to a four-wheel drive system and thereby minimize wheel slippage. Thus, an all-wheel drive system possesses several advantages but suffers from the standpoint of relatively poor fuel economy.